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Chapter 0

Bio-Inspired Photonic Structures:Prototypes, Fabrications and Devices

Feng Liu, Biqin Dong and Xiaohan Liu

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/50199

1. Introduction

Like the ability of electron regulation of electronic semiconductors, the photonic analogsusually considered as photonic structure materials are regarded as essential for lightmanipulation [1, 2]. With particular designs of photonic structures, they are expected toachieve different far-field and near-field optical features and thus lead to a perspective inall-optical circuit [3]. Though humankind has entered the nano-scale realm several decadesago, it is still a hard task for engineers to explore novel optical functional devices due to thelimited experiences and originalities on artificial photonic structures design and the desiredoptical features. Additionally, it is also great challenges to fabricate photonic structures, owingto their sub-optical-wavelength to sub-micron featured sizes, especially in a high dimensionalway by nano-fabrication technologies today.

By contrast, nature are found to develop photonic structures millions of years before our initialattempts. Diversified photonic structures, most of which are sophisticated and hierarchic,are revealed in beetles, butterflies, sea animals and even plants in recent surveys [4–10].The exhibited optical features are regarded to have particular biological functions suchas signal communications, conspecific recognition, and camouflage, which are optimizedunder selection pressure. Naturally, the occurring photonic structures provide us ideal‘blueprints’ on design and stimulate similar optical functional devices. Various fabricationmethods of bio-inspired photonic structures are explored [11–16]. By chemical methods(e.g. Sol-Gel, colloidal crystallization, chemical systhesis), nanoimprint lithography andnanocasting, physical layer deposition (PLD), atomic layer deposition (ALD), and etc.,bio-inspired photonic structures, their reverse counterparts, and applications are achievinggreater success than ever before.

This Chapter will review the typical bio-inspired photonic structures and focus on thebiomimetic fabrications, the corresponding optical functions and the prototypes of opticaldevices. It is organized as follows: four subsections are introduced in Section 2, in which eachdescribes one catagory of bio-inspired optical functional devices. In every subsection, thenature prototype is introduced first, then followed by biomimetic fabrication methods and

©2012 Liu et al. , licensee InTech. This is an open access chapter distributed under the terms of theCreative Commons Attribution License (http://creativecommons.org/licenses/by/3.0),which permitsunrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Chapter 6

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Figure 1. Antireflection structures. (a) Hexagonal arranged tapered pillars root in the surface of cicadawings in top view (tilted view of the pillar array is showed in the inset) [19]; (b) The tapered pillars leadto gradual refraction index variation in view of effective theory and then minimize the reflectionaccording to Fresnel relations.

optical features of artificial analogs, finally closed on the bio-inspired optical devices. A briefperspective is given in Section 3.

2. Bio-inspired optical functional devices

2.1. Anti-reflection devices

2.1.1. Prototypes

In arthropodal animals such as butterflies, nipple arrays with typical spacing of opticalwavelength or subwavelength are commonly found on surfaces of their compound eyes,which are believed helpful to the light-harvest efficiency of the biological visual system[17]. With optical impedance matching to the ambience, the light transmission are enhanced.Another analogous examples are the transparent wings of some lepidoptera insects likehawkmoths [18] and cicada [19] (Fig.1 (a)). The tapered pillars lead to gradual changes ofrefraction index in view of effective medium theory (Fig.1 (b)) and therefore play a key rolein minimizing the reflection over broadband and large viewangles. In order to physicallyexplain the anti-reflection origin, the Fresnel equations are given as follows.

rs = |n1cosi1 − n2cosi2n1cosi1 + n2cosi2

|, (1)

rp = |n1cosi2 − n2cosi1)(n1cosi2 + n2cosi1

|, (2)

where rs and rp refer to reflection coefficients of s polarised light (the electric field of the lightperpendicular to the incident plane) and p polarised light (the electric field in the incident lightplane), n1 and n2 are refraction indices of neighboring mediums, respectively. The relationshipbetween incident angle i1 and refraction angle i2 is given by Snell’s Law n1sini1=n2sini2. Fromthe equations, e.g., for normal incidence, rs and rp are suppressed to near zero if n1 and n2have very close values [20, 21].

2.1.2. Bio-inspired fabrications

Inspired by nature, many efforts are made in exploring techniques for fabricatingthe anti-reflection nanostructures and a variety of methods, e.g., conformal-

108 Optical Devices in Communication and Computation

Bio-Inspired Photonic Structures: Prototypes, Fabrications and Devices 3

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Figure 2. Improved antireflection of biomimetic nanotips by ECR plasma etching technique [25, 27]. (a)Schematic diagram for the silicon nanotip formation on silicon wafer; (b) and (c) show a tilted top viewand a cross-sectional view of silicon nanotips, respectively; (d) Compared with polished silicon wafer(left), 6-inch silicon wafer coated with silicon nanotips (right) show greatly improved broadband andquasi omnidirectional anti-reflection.

evaporated-film-by-rotation (CEFR), colloidal lithography, self-masked dry etching,nanoimprint lithography (NIL), ALD and other approaches, are realized [16].

Oblique angle deposition (OAD) is usually employed to fabricate anisotropic film which isoriginated from the oblique growth of contained nanorods with a tilted angle to the substratesurface normal. The so-called CEFR method, which rotates the substrates at a high speedunder OAD, leads to the straight growth of a dense array columns to substrate surface ratherthan helical structures which are formed under low rotation rate. That is, CEFR method issuitable for conformal replication of the photonic structure with a curved surface even underthick film deposition. With compound eyes of the fruit fly as bio-templates, it is reportedartificial replica is successfully fabricated and hence similar optical features are inherited,respectively [22].

Many lithography techniques are applied for fabricating antireflection structures, amongwhich colloidal lithography is a much simpler approach [23]. With colloidal crystals as masks,the silicon substrate is etched by reaction ion etching (RIE). During the fabrication duration,the colloidal spheres are etched by RIE gradually firstly, leading to a reduced transverse crosssection of the spheres and thus an increasing exposure of the substrate. Attributed to thefeatures of RIE, the etching rates of the apex and the junction parts of the spheres are notuniform, resulting in the etching morphology modification from frustum to cone arrays onthe substrate finally [24].

Electron cyclotron resonance (ECR) plasma etching technique is employed by researchers tofabricate antireflection structures with much higher aspect ratio surfaces [25–27]. With theselected gas-mixture consisting of SiH4, CH4, Ar, and H2, one step and self-masked dryetching are realized for fabricating high density nanotip arrays on a 6-inch silicon wafer. Thefabrication progress is illustrated in the schematic diagram of Fig. 2(a). In brief, SiC clusters,which size and density can be tuned via process temperature, gas pressure, and composition,are formed on surfaces of the silicon substrate due to the reaction of SiH4 and CH4 plasma.

109Bio-Inspired Photonic Structures: Prototypes, Fabrications and Devices


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Figure 3. Cicada wing structure fabrication by NIL [19]. (a) Schematic diagram of NIL using cicadawings as bio-templates; (b) The fabricated structures and (c) the natural photonic structures show similarmorphology from the SEM images.

Ar and H2 are responsible for the dry etching process. The SiC clusters then act as nanomasksor nanocaps to protect the underlying substrate from etching, thus forming an aperiodic arrayof silicon nanotips with their lengths varying from ∼1000 nm to ∼ 16 μm finally (Fig. 2(b)and (c)). Even superior to natural prototypes, the biomimetic antireflection structures exhibitstriking omnidirectional low reflection shown in Fig. 2(d) over a broad range of wavelengthsfrom ultraviolet to terahertz region, irrespective of polarization.

Avoiding time-consuming and complicated mask fabrication, scientists also attempt todirectly use cicada wings or insect eyes as bio-templates [19, 28, 29]. For example, the nipplearrays on wing surfaces are stamped under certain pressure on glass-phase PMMA, which isat higher degree than its glass-transition temperature, supported by silicon wafer. A releaseprocess makes the polymer reverse nanostructures of the bio-templates. With the patternedPMMA as a mask or a mold, inverse or similar structures of the cicada wing are achievedby RIE or thermodeposition. A schematic diagram of the NIL, the fabricated structures, andthe natural templates for comparison are illustrated in Fig. 3(a), Fig. 3(b), and Fig. 3(c),respectively. It is also worthy to note that an extra advantage of using bio-templates in NILprocess is the notable low-surface-tension, which is vital for the release process, due to thewax layer commonly found on surfaces of plants and insects.

Taking advantages of accurate thickness control and three-dimensional (3D) fabrication ofALD, conformal replica is accomplished after ALD growth and sintering the hybrid structureswith fly eyes as bio-templates, achieving similar anti-reflective features in the artificial analogfinally [29].



(a) (b)

(c)

(d)nanodome solar cells thin film solar cells

(e)

(f)

TCO a-Si:H

Ag

Figure 4. (Color online) Solar cells coating antireflection structures [32, 34]. (a) Nanodome solar cells intop view. Scale bar 500 nm; (b) Schematic diagram of the cross-sectional structures of the solar cells; (c)The photographs of nanodome solar cells (left) and flat film solar cells (right); (d) Dark and light I-Vcurve corresponding to (c); (e) and (f) The optimized double-sided nanostructure yields a photocurrentclose to the Yablonovitch limit at an equivalent thickness of 2 μm.

2.1.3. Potential applications

Due to the high reflectance of silicon solar cells (more than 30%) induced by the high indexcontrast of silicon and air according to the Fresnel equations, scientists are already aware ofthe vital roles of high-quality antireflection coatings at early ages of solar cell fabrications [30].Inspired by antireflection structures found in moth eyes, nanodomes or similar architecturesare reproduced on surfaces of solar cells, leading to a dramatic light absorption increase andtherefore a superior efficiency improvements than that of quarter-wavelength antireflectioncoating [31–33], as shown in Fig. 4(a)-(d). The recent theoretical investigations even reporta high light trapping close to the Yablonovitch limit in the silicon solar cell by optimizing adouble-sided antireflection structure design (Fig. 4(e) and (f))[34].

The biomimetic antireflection structures can also play a key role in light extraction oflight-emitting devices (LEDs) [35–37]. Because of the total internal reflection and thewaveguiding modes in the glass substrate, only about 20% amount of the generated lightcan irradiate from the LEDs. By fabricating silica cone arrays on the surfaces of the ITOglass substrate to modulate the above two bottleneck factors, the light luminance efficiencyof white LEDs is significantly improved by a factor of 1.4 in the normal direction and evenlarger enhancement for large viewing angles.

Another fascinating application of the inspired antireflection structures is the use in the microSun sensor for Mars rovers [38]. On the basis of the recorded image by an active pixel sensor,the location coordinates of the rover can be calculated. However, the ghost image originatingfrom the multiple internal reflection of the optical system leads to severe limitation of theaccuracy. By fabricating dense nanotip arrays on the surfaces of the sensor, the internalreflection is minimized to be nearly 3 orders of magnitude lower than that of no treatments,resulting in a more reliable three-axis attitude information.



d variation

(c)

(d) (e)

θ1 θ2

platelet(f)

(g) (h) (i)

θ variation

n variation

(a) (b)

Figure 5. (Color online) Color tuning mechanisms found in nature [50, 53, 56]. The coloration of (a)longhorn beetles, (d) iridophore of tropic fish neon tetra, and (g) hercules beetles can reversibly changetheir coloration to (b), (e), and (h), which are intrinsically induced by (c) period d, (f) tilting angle θ, (i)refraction index n variation, respectively. Scale bars: (a) and (b) 10 mm, (d) and (e) 20 μm.

2.2. Color-tunable devices

2.2.1. Prototypes

Besides the well known coloration change strategy via migrations and volumes change ofpigment granules such as chameleons, nature develops a second approach which is known asstructural coloration change (SCC). By varying photonic structure characterizations, incidentlight angle, or the refraction index contrast of the color-produced optical system via theenvironmental stimuli, reversible coloration changes, which are basically passive, are revealedin fishes, beetles, and birds [39–46]. Most structural basis of SCC are attributed to theone-dimensional (1D) reflectors. For example, the damselfish Chrysiptera cyanea can changeits color from blue to ultraviolet rapidly under stressful conditions, which is triggered bythe simultaneous change in the spacing of adjoining reflecting plates made of guanine in theiridophore cell [47–49]. In insect world, the coloration change of longhorn beetles Tmesisternusisabellae from golden to red is revealed to originate from the swollen multilayer after waterabsorption [50] (Fig. 5(a), (b) and (c)). Another origin of structural coloration change is thetilted angle variability of the nanoplates with respect to incident light, which is found in tropicfish neon tetra Paracheirodon innesi [51–53] (Fig. 5(d), (e), and (f)). Physically, the underlyingmechanism of the coloration change in the mentioned 1D biological photonic structures canbe understood according to the given formula

λmax = 2(n1d1cosθ1 + n2d2cosθ2), (3)

where d is the layer thickness, n is refraction index, and θ is angle of refraction. The subscriptsrepresent the layer index. The angle of refraction at different layers θ1 and θ2 can be obtainedfrom Snell’s law n1sinθ1=n1sinθ1=sinθ0, where θ0 is the incident angle from air. From theequation, it is easy to elucidate that either n, d, or θ0 is related to the optical path in themultilayer and thus leads to shifting interference peaks at different wavelengths.

In biological system, high-dimensional photonic structures responsible for the colorationchange are also discovered, though they are rare. An intriguing example is hercules beetles



(a) (b)

(c) (d)

Figure 6. (Color online) Structural color printing using ‘M-Ink’ [57]. (a) Three-phase material system of‘M-Ink’; (b) ‘M-Ink’ particles align in a chain under external magnetic field, which acts as basic colorationunits of color pattern after fixing by UV light; (c) Reflection images of multicolored structural colors(upper) and transmission photographs of the same sample (below) by gradually increasing magneticfields; (d) High-resolution multiple structural color patterns. Scale bars: (b) 1μm, (c) and (d) 100 μm.

Dynastes hercules which can alter their appearance from khaki-green to black providing theambience changes from dry to a high humidity level [54–56] (Fig. 5(g) and (h)). The 3Dphotonic crystal structures (nanoporous structures) are filled with water instead of air voidsin dry status, rendering different refraction index contrast and thus the variation of Braggscattering (Fig. 5(i), here only 2D cross section is illustrated). However, the underlying physicsof coloration change induced by high-dimensional photonic structures is no other than that oftheir 1D counterparts.

2.2.2. Bio-inspired fabrications and applications

Due to the obvious appearance changes which are easy to be picked up with the nakedeye, color-tunable devices are explored to identify the status changes by temperature, vapor,solvent, humidity in ambience, the applied mechanical force, electric field, magnetic field andetc. Besides the sensors, some novel writing system (’paper and ink’) are developed. Thekey idea is to modify period (d), refraction index (n), viewangle (θ) or their combinations ofphotonic structures, just like nature shows us.

2.2.2.1. d variation

Because of relatively simple control by the environmental stimuli and large spectral variationwhich can be recognized by the naked eye, approaches on the modulation of photonicstructure period are always of scientist interests in obtaining tunable color applications.

’M-Ink’ is a mixture of colloidal nanocrystal clusters (CNCs), solvent and photocurable resin(Fig. 6(a)). With the superparamagnetic Fe3O4 nanocrystals encapsulated by silica shell,’M-Ink’ can response to external magnetic fields. The role of the resin is to provide repulsiveforce which balances the attractive force of the CNCs. Without external magnetic fields,CNCs are randomly dispersed (infinite period) in liquid resin. The exhibited colorationis consistent with the magnetite, to be brown. After applying magnetic fields, the CNCsare assembled to form chain-like structures along the magnetic field lines (Fig. 6(b)). Theadditional magnetic force, the intrinsic force among the CNCs, and the repulsive force by resin



establish dynamic balance with variation of the external magnetic fields, tuning the distancebetween the neighboring CNC (finite period). The switchable period then determines thecolor of the light diffracted from the CNC chain, leading to a full color show (Fig. 6(c)).The final step is to fix the desired coloration. After exposure to ultraviolet (UV) light atdifferent exerted magnetic fields locations, the chain-like CNCs can be frozen in the solidifiedresin instantaneously, remaining the periods of the chains undistorted and accomplishinghigh-resolution color pattern fabrication (Fig. 6(d)) [57].

With similar principle, the electric field-driven tunable color sensor is also realized by highlycharged polystyrene (PS) colloids which form non-close-packed face-centered cubic (fcc)lattice [58]. Tuning the period along [111] direction by the balance of the exerted electrostaticforce and the repulsive force, the exhibited coloration changes as a result of the applied electricfield. The so-called ’P-Ink’ is an electroactive material which consists of inverse opal insidepolyferrocenylsilane (PFS) derivatives matrix. Such ink fabrication includes 3 primary steps:An opal film made of silica spheres is deposited onto glass substrate first by self-assembly; UVlight is exposed to the sample in order to solidify the matrix and then form a stable PFS/silicacomposite; With diluted HF, inverse opal structure are realized in the elastomeric polymermatrix. By applying tunable voltage, macroscopic swelling and shrinking of the polymermatrix and microscopic Bravais lattice change responsible for the reverse coloration occur [59].Stimulated by electrical forces, quite a few switchable coloration devices or sensors based onother materials or circumstances can be found elsewhere [60–62].

Many pressure-based photonic and even laser devices are reported [63–66]. Usingmonodispersed PS spheres to form cubic close packing (ccp) structures which are embeddedin polydimethylsiloxane (PDMS) matrix, reverse colors are observed simply by stretching andreleasing the rubber sheet. Upon mechanical stress, the lattice is elongated along the appliedforce direction, while the interplanar spacing in the perpendicular direction (i.e. distancebetween the (111) planes) decreases because of the nearly invariance volume of the rubber.The compressed distance leads to a blue-shift, e.g., from red to green [63]. Such opal rubber isbelieved to have practical applications such as a color indicator, tension meter or elongationstrain sensor. The inverse opal structures (filled by air voids) in elastomer network arealso synthesized [64, 67] (Fig. 7(a)). The porous elastomeric photonic crystals (EPCs) showhighly reversible optical response to compressive force, e.g., 60 nm spectral blue-shift undera compressive pressure of ∼ 15 kPa in the structures having 350-nm void size. Although thecoloration change can be attributed to the Bravais lattice deformability, like the mechanismmentioned before. However, it is especially noteworthy that porous EPCs remain nearlyundeformed in orthogonal directions when an external pressure is exerted in one direction,which can be ascribed to the high filling factor of air voids. The air voids enable the distortionof the cross-sectional void spaces from roughly circular to elliptical shape and a reduction ofthe air volume fraction under pressure. The elastic deformation feature of such structureshelps to reduce the redistribution of stress along lateral directions when compressed by apatterned surface, leading to novel biometric applications such as the fingerprint recognitiondevices, as shown in Fig. 7(b). Additionally, air voids of porous EPCs provide a platform toincorporate with other functional materials for us to explore new applications. For instance,filling PbS quantum dots in the air voids, the photoluminescence (PL) emission, which leadsto many potential applications in the near-infrared region, can be modified by overlappingwith the forbidden bandgap of the inverse opal structures (Fig. 7(c)).

The chemical solvents are also used to be as stimuli. An interesting example is inventionof new type ’photonic paper/ink’ system [68, 69]. With novel soft materials consisting of



(a)(b)

(c)

Figure 7. (Color online) From color fingerprinting to the control of photoluminescence in EPC films [67].(a) Schematic diagram of the elastic inverse opal structure fabrication; (b) A captured still image of theEPC film under compression by an index finger; (c) NIR-emitting PL emission of colloidal PbS quantumdots which are incorporated into voids of the EPC can be tuned by overlapping the forbidden gap (solidblack line) with the PL peak (grey filled curve).

closely packed PS and polymer elastomer (colloids and PDMS), the exhibited coloration canbe altered reversely by immersing the materials into silicone liquid (’writing process’) andan evaporation process (‘erasing process’). The spacing between the (111) planes is adjustedby the strength of interaction between PDMS matrix and the contained silicone oligomerswith different molecule weight in the liquid. With different solvents (’ink’), the swelling andshrinkage of the matrix show a featured reversible shift of Bragg diffraction peak. A multilayerbased on alternating Teflon-like layer and Au nanoparticle/Teflon-like layer composite layeroperates not in visible but optical telecommunication wavelength range [70]. When thestructure is exposed to different organic solvent vapor (e.g. acetone, ethanol, methanol, water,chloroform, and etc.), the molecules enter the metal/polymer composite inside the holes andmicrovoids in its structure, resulting in the swelling up, e.g., for acetone vapors at a molarfraction of 0.25, with an relative increase of 12.5% in thickness to an equilibrium state whileleaving the Teflon-like layer unchanged due to its inert chemical features. The measuredreflectance show a large variation of 0.2 μm, which is advantageous to the detection of theorganic/inorganic vapors.

Other external stimuli such as UV light, heat, or chemical reaction are applied to trigger andfabricate coloration sensitive materials as well by reversely controlling the spacing of theresponsible photonic structures. Detailed information can be found in some references andrecent reviews [14, 15, 61, 71–74].

2.2.2.2. n variation

Besides d, the refraction index n is another attribute of photonic structures. Differentapproaches to alter refraction index, in which infiltration is most regular, are revealed inorder to achieve novel visual applications. Inspired by beetles D. hercules, 3D nanoporousstructures are fabricated by so-called dip-coating deposition, achieving humidity sensing [75].The 3D architecture, which is inverse opal structure, is reported to have its reflective peak



(a)

(b)

(c) (d)

Figure 8. (Color online) Silica inverse opal films of ‘Watermark-Ink’ system [76]. (a) Schematicprocedure of chemical encoding; (b) SEM images of the fabricated inverse opal structures incross-sectional view (left) and top view (right); (c) Optical images of the fabricated film in which theword ‘W-INK’ is encoded via functional chemical groups on surfaces. (d) Optical images of differentencoded patterns under different solvents.

red-shifting 14 nm when the relative humidity changes from 25% to 98%. It is worth notingthat after treatments by O2 plasma, the 3D photonic structures exhibit large bandgap shiftof 137 nm and dramatic coloration change from bluish green in dry state to red in fully wetstate. The modified hydrophilic feature play an important role in water collection in air voidsat different level of humidity, leading to larger variation of refraction index contrast. Theother clew for the remarkable coloration change can be ascribed to the high filling factor ofair voids of inverse opal structures. The high value of ∼ 76% results in a wider modulationof refraction index contrast by the amount of solvent absorption. Tuning hydrophilic featuresby chemical groups, the inverse opal film can even be used as ‘Watermark-Ink’ system [76].In the studies, the inside porous structures are functionalized with chemical group (R1,R2, R3, or R4) through vaporizing an alkylchlorosilane. The chemical functionalities of thestructures then are erased and the surface is reactivated by O2 plasma exposure, leaving apatterned functionalized region where is masked by a PDMS polymer. With iterations ofsuch kind of functionalization and reactivation, the opal structures are locally patterned bydifferent chemical groups, leading to differentiate hydrophilic feature (Fig. 8(a) and (b)).When immersing the film into specific fluids, regulated infiltration by the wettability occursspatially in the film and thus surveys different optical responses (i.e. different patterns, Fig.8(c) and (d)). The fabricated structures are expected to have applications in encryption as wellas colorimeter.

Besides infiltration, phase-transition materials can also induce refraction index variation andswitchable coloration, providing the transition conditions are satisfied. The best-knownphase-transition material maybe is liquid crystals (LCs). Above the phase-transitiontemperature of 34◦, LCs change their nematic phase which is anisotropic to isotropic phase,leading to a significant change of refraction index and coloration, e.g., in inverse opalstructures. By mixing the active material into LCs, sensitive reflection and polarizationtriggered by UV light are reported by a series of subsequent researches [77–79]. Besides LCs,various sensitive materials, including Ag2Se, WO3 and ferroelectric ceramics, are also foundfunctionally in incorporating with diversified photonic structures to obtain tunable structuralcoloration materials and thus fabricate thermo- or electro-sensors [80–83].

2.2.2.3. θ variation

Iridescence is a characteristic of structural coloration, which is determined by banddispersions of photonic structures. With various incident angle or observation direction, the



(a)

(b)

(e)

(f)

(c)

(d)

Figure 9. (Color online) Magnetochromatic microspheres switched between ‘on’ and ‘off’ states byrotating external fields [84, 85]. Schematic illustrations, optical images, and the corresponding SEMphotographs of ‘on’ and ‘off’ states are (a), (c), (e), and (b), (d), (f), respectively.

perceived coloration is changing. Hence, it is feasible to explore novel tunable color devicesby tuning orientations of the photonic structures [84–86]. Just like the fish neon tetra P. innesi,magnetochromatic microspheres alter their appearance by tilting the inner photonic structureswith respect to the incidence (Fig. 9). As mentioned before, with superparamagnetic Fe3O4particles coated by silica in PEGDA emulsions, the applied magnetic field guides to formphotonic chains, in which the interpaticle distances depend on the balance of the attractive andrepulsive force. Polymerized by UV light, microspheres are solidified and the inner periodicphotonic chains, which give rise to structural coloration, are fixed permanently. Dispersingthe microspheres into liquids, various colors are observed in top view due to the randomorientations of the photonic chains in the microspheres. By applying external magnetic fields,the microspheres tend to rotate in order to keep the magnetic chains inside in the directionof the magnetic vector, leading to a homogenous coloration of green exhibited. The featureof the switchable coloration ’on’ and ’off’ status by the external stimuli is believed to haveapplications in color display, signage, bio- and chemical detection, and magnetic field sensing.

2.3. Structural color mixing and applications

2.3.1. Prototypes

Structural coloration results from the interaction of light and photonic structures with featuredsize of visible wavelengths. It is even more widespread than pigmentary coloration in animalworld. Some literatures have well reviewed the field comprehensively. In the Chapter, we donot plan to pay attention to the overall structural coloration but only focus on a specific subject’structural color mixing’ [87–92]. In tiger beetles Cicindela oregona(Fig. 10(a)-(c)) [87, 88], thehoneycomb-like pits are found on surfaces of the elytra. Under microscope, the brown morphactually includes blue-green patches in the red background, while the black one consistsof magenta patterns surrounded by dull green. The microscopic colors are the results of



5 μm

100 μm

12 μm

10 μm

(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

10 μm

Figure 10. (Color online) Structural Color Mixing in Nature [87, 90, 91]. Photographs, opticalmicroscopy and SEM images of tiger beetles C. oregona ((a), (b), (c)), long-jointed beetles C. obscuripennis((d), (e), (f)), and swallowtail butterflies P. palinurus ((g), (h), (i)), respectively.

light interference by the multilayer structures in the cuticle. However, due to the coloredpatches (40-80 μm across) are too small to be resolved by the unaided eye (the resolution d isdetermined by the Rayleigh criterion d=l×0.61λ/D, where l is distance of distinct vision, λ thewavelength and D the average pupil diameter of humankind), the perceived coloration is amixture of the discrete colors, leading to a totally different exhibition from that equipped withmicroscope. From the investigations of photonic structures of beetles Chlorophila obscuripennis(Fig. 10(d)-(f)), we revealed the color is a juxtaposition in a smaller region (∼ 10×10 μm2) bygreen on the ridges and cyan in centers of the pits. Furthermore, the pits on the elytra surfacegive rise to diffused light reflected over a wide large of angles, leading to inconspicuouscoloration shown [91]. In butterflies Papilio palinurus (Fig. 10(g)-(i)), the scanning electronicmicrographs show surfaces in the scales comprise a similar two-dimensional (2D) pits (4-6μm in diameter and 3 μm at the greatest depth). The flat regions between and in pitsappear yellow, and the inclined region contributes to blue color. Because of the limitationof human eyes’ resolution, the butterfly displays a mixture color of green, i.e. yellow plusblue turns to be green. Due to sufficient depth of the pits, light which normally incidentson the inclined side can experience dual reflection and is back-reflected. The retro-reflectionis found to play a key role in the polarization conversion of incident light, which is crucialin some novel optical applications. Thanks to symmetric feature of the pits, no macroscopicpolarization effects can be observed [90]. On the cover scale surfaces of butterflies Sunevecoronata, however, the natural occurring triangular grooves array not symmetric but in 1Dway with a period of ∼ 2 μm, giving rise to polarization conversion of the normal incidentlight in a specific direction. Intriguingly, it is revealed the coloration macroscopically remainsunconscious change under different polarized illumination. The reason is the curly scales,which induce polarization conversion between the neighboring rows of scales but in the



(a) (b) (c) (d)

(e) (f) (g)

Figure 11. (Color online) Mixed structural coloration and applications inspired by nature [95]. (a)Schematic diagram of artificial samples mimicking the scale structures of P. blumei; (b) SEM photographsin top view show the concavities on surface of the replica which is (c) green macroscopically but (d)resolved yellow and green microscopically; (e) Modifications of the concavities morphology by meltingcolloidal spheres embedded in the concavities lead to a striking change in color of the sample from blueto red viewed (f) in direct specular reflection and (g) in retro-reflection. Scales: (a) 2 μm, (c)(f)(g) 5 mm,and (e) 5μm.

orthogonal direction. The nanostructures having different magnitude in size therefore cancelout the macroscopic polarization effects (Fig. 12(a)-(d)) [89].

2.3.2. Bio-inspired fabrications and applications

Structural coloration may be especially crucial for future color and related industry because ofthe non-fading feature (if the photonic structures are undeformed) [93, 94] and environmentalfriendliness. Naturally, structural coloration mixing inherits the advantages. Mimicking thenature, mixed structural color and its application can be obtained. For example, PS colloidswith a diameter of 5 μm are assembled on a gold-coated silicon substrate. A 2.5-μm-thick layerof platinum or gold is then deposited to fill the interspaces of the colloids by electrochemicalapproach, creating a negative replica. After removal of the PS colloids by ultrasonic waves anda sputtering thin carbon film, a multilayer of quarter-wave titania and alumina films is grownby ALD (Fig. 11(a)), inheriting the morphology of hexagonally arranged pits of the negativereplica (Fig. 11(b)). The sample exhibits similar color mixing with that of butterflies P. blumei(Fig. 11(c) and (d)). Moreover, by modifying the surface morphology of the imitation (Fig.11(e)), even visual information, e.g., a picture, can be encoded into the photonic structureswhich display a striking appearance change from pale blue in the specular direction to red inretro-reflection (Fig. 11(f) and (g)). The bio-inspired work is expected to find applications insecurity labelling field or color industry such as painting and coating [95].

Inspired by color mixing researches, some novel applications based on polarizationconversion are designed [89]. For instance, by etching periodic triangular-like grooves onsurfaces of a flat multilayer, the film displays a coloration of green, which is actually a mixedcolor from yellow in the flat region and blue in the grooves in normal incidence, as shownin Fig. 12(e). Because of the broken symmetry of the 1D structures, the blue color can besuppressed by using a polarizer on light path, leading to the exhibited coloration change to



(a)

(b)

(c)

(d)

(e)

(f)

Figure 12. (Color online) (a) Optical image of a male S. coronata butterfly in dorsal view; Theappearances under (b) TM and (c) TE polarization light show distinguished difference of the bluecomponent, which can be attributed to (d) the broken symmetry of the triangular grooves in the surfaceplane; Two kinds of anti-fake or encryption designs, which apply (e) colored effect or (f) grey levelchange linked to the polarization and the broken surface symmetry, are inspired [89].

yellow (i.e. the hue in flat regions). That is, the grooved areas turn the light polarizationπ/2 and thus are shadowed by the inserted polarizer. Following the ideas, an encodedpattern which comprises grooved areas can be distinguished with the background consistingof perpendicular grooves via the luminosity level under different polarized light incidence(Fig. 12(f)). The two imaged examples inspired by the grooved structures of the butterflyshow us great prospective in fields such as anti-fake and encryption.

2.4. Other bio-inspired fabrications and applications

Besides the main categories, some other bio-inspired photonic applications are also reported[96–100]. For examples, the natural scales of Morpho sulkowskyi are extremely sensitive tothe different environmental vapors, which lead to dramatically improved responses as unitsof potential sensor applications compared with that of current devices[101]. Using blackscales of the butterfly Papilio paris and Thaumantis diores as templates, hierarchically periodicmicrostructure titania replica was systhesized by chemical procedures. The high surface areainherited from the natural templates is great advantages to the light harvesting efficiency anddye sorption when the replica is used as photoanode in dye-sensitized solar cell [100]. WithMorpho as bio-templates, alumina replicas show the potential applications in waveguidesand beam-splitters under thin film coating by ALD and sintering [97]. Under thick filmcoating, the hybrid structure is found not to inherit the ’Christmas-tree’ structures butdevelop a ’Pyramid-like’ structures which have potential applications in light trapping [99].With the complex photonic structures (multilayer plus 2D amorphous structures) of beetlesTrigonophorus rothschildi varians as ‘blueprints’, the artificial counterpart which is fabricated byFIB etching holes through a 5×SiO/SiGe multilayer structure show similar optical featureslike non-specularity and only slightly angle-dependent reflectance [98]. Further bio-inspiredwork is undergoing which is stimulated by a recent revealed 3D architecture [102], aiming



at the ultra-negative angular dispersion of diffraction and potential novel dispersive opticalelements.

3. Perspectives

In the Chapter, several important kinds of bio-inspired photonic applications are reviewed,including antireflection devices, color-tunable sensors, structural color mixing applicationsand etc. The nature nourishes scientists the functional optical applications either theblueprints of photonic architecture or directly the bio-templates. Due to the higher indexof inorganic materials used, the mimicking photonic structures even show better opticalperformances as well as enhanced mechanical properties of high temperature tolerance,stability and infrangibility. The biomimetic applications are anticipated to help our life betterin the near future. However, complicated photonic structures (e.g. those of high-dimensional,hierarchic, amorphous features in nature) still remains hardly reproduced or, if they arefabricated successfully, the efforts involved are so great using the traditional fabricationways that optical devices can not commercially explored. Thorough physical mechanismunderstanding as well as better fabrication approach explorations may help to simply thestructure fabrications, achieve similar optical functions and realize commercial applications.In addition, adding substances such as functional chemical groups, fluorescence particles,metal, or other active materials, the mimicking photonic structures allow the properties ofinterest to be augmented, which may open a new window of novel optical device exploration.Although the photonic biomimicry is in its infancy, we believe that the bio-inspired opticaldevice would surely have profound impacts on our modern society.

Author details

Feng LiuLaboratory of Opto-electrical Material and Device, Department of Physics, Shanghai NormalUniversity, Shanghai, China

Biqin DongDepartment of Mechanical Engineering, Northwestern University, Evanston, USA

Xiaohan LiuDepartment of Physics, Fudan University, Shanghai, China

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